The Materials Used in Timber Engineering
1.7 ADHESIVES USED IN TIMBER ENGINEERING .1 General
fasteners are usually fabricated from hot-dip zinc-coated steel sheet or coil to BS EN 10147 or austenitic stainless steel to a grade defined in BS EN 10088-2. The thickness of the plate should be not less than 0.9 mm and not more than 2.5 mm.
A zinc coating should be equivalent to or better than Z275 of BS EN 10147.
The majority of plates produced have integral teeth pressed out at right angles to the plate to give teeth at close centres which will subsequently be pressed into the timber by specialist machinery. The strength capacities of the various nail configurations are determined by test and published in the plate manufacturer’s Agrement certificate. As an alternative, plates with pre-punched holes through which treated nails are driven can be used. The principles of BS 5268-2 with regard to nailed joints and steel plates may be used to determine the strength of a joint.
Joint slip will take place as with any nailed joint. For joint slip characteristics of a particular plate the manufacturers of the plate should be consulted.
1.6.11 Other fasteners, gussets and hangers
Rag bolts, rawl bolts, ballistic nails, etc., all have their use with connections for timber. Steel gussets or shoe plates, etc., can be used with timber in a similar way to that in which they are used with structural steelwork, except that in this case nails and screws can be used as well as bolts.
In recent years the use of proprietary I beams has brought an increased range of joist hangers, eaves and ridge supports and hangers, indeed methods of jointing timber members for almost any imaginable situation (for example, seismic condi- tions). These products are the result of extensive testing, confirmed by use in practice. Obviously the finish of any fastener, hanger, etc., must be suitable for the service conditions.
1.7 ADHESIVES USED IN TIMBER ENGINEERING
requirements of BS EN 391. These glues are normally gap-filling thermosetting resorcinol–formaldehyde (RF), phenol–formaldehyde (PF) or phenol/
resorcinol–formaldehyde (PF/RF) resin adhesives. They are required where there is high hazard from weather, temperatures approaching 50 °C or higher, high humidity, or a chemically polluted atmosphere. They are often used (sometimes with a filler) for structural finger jointing. These glue types require careful quality control during storage, mixing, application and curing, and are therefore normally intended for factory use.
1.7.3 Boil-resistant or moisture-resistant glues
When a component is unlikely to be subjected to any serious atmospheric condi- tions once in place, and is only likely to receive slight wettings during transit and erection, it may be possible to use an adhesive which matches the ‘boil resistant’
(BR) type formerly defined in BS 1204: Part 1: 1979. These are normally gap- filling melamine/urea–formaldehyde (MF/UF) or certain other modified UF adhesives which can match the BR or MR requirements. They are usually cheaper than WBP adhesives. For certain low hazard situations it might be possible to use an adhesive which matches only the ‘moisture-resistant and moderately weather-resistant’ (MR) requirements of BS 1204: Part 1. These are normally urea–formaldehyde (UF) adhesives, perhaps modified, to match the MR require- ments. They require careful quality control during storage, mixing, application and curing, and are therefore normally intended for factory use.
1.7.4 Interior glues
A structural component is not normally bonded with an adhesive which matches only the ‘interior’ (INT) requirements of BS 1204: Part 1. There are exceptions, perhaps where a site joint is required or where control of the gluing cannot be guaranteed to a sufficient degree to enable a WBP, BR or MR glue to be used. An
‘interior’ glue should be used only when the designer can be certain that the glue joint will not be subjected to moisture or high temperature in place.
A typical non-structural adhesive commonly available in the market place is polyvinyl acetate (PVA). This adhesive is subject to high creep and deterioration in a moist environment. Developments of the internal bonding characteristics of PVA have led to the ‘cross-linked PVA’ that has good weathering characteristics but is slightly more flexible (‘rubbery’) than the traditional wood adhesives. It therefore has application for joinery rather than structural members but it is an indication of future adhesive development and the reasons for moving towards performance specifications.
1.7.5 Epoxy resins
Epoxy resins are finding increasing application in timber engineering, particularly in the bonding of steel and timber. They can be extremely useful in localized
The Materials Used in Timber Engineering 39
situations such as ensuring true bearing in a compression joint between timber and a steel bearing plate or, for example, in sealing the possible gaps between a timber post and a steel shoe. To reduce the cost in such compression joints the resin can be mixed with an inert filler such as sand.
1.7.6 Gluing
Gluing, including gluing of finger joints, is described in more detail in Chapter 19.
Basically the quality control requirements to obtain a sound glue joint are correct storage, mixing and application of the adhesive, correct surface conditions, mois- ture content and temperature of the timber, and correct temperature of the manu- facturing environment during application of the adhesive and during curing. The instructions of the adhesive manufacturer must be followed with regard to adhe- sive mixing, pot life, open assembly time, closed assembly time, setting and curing periods. The joints must be held together during gluing, either by externally applied pressure (usually necessary where solid timber is being glued in order to achieve close contact – the pressure for softwoods is usually quoted as 0.7 N/mm2) or by nails or staples (glued–nailed construction for bonding sheet material to timber with the fasteners left in, although their only duty is to hold the surfaces in close contact during curing, the design shear force being taken only by the adhesive).
Adhesives and the use of adhesives must be compatible with any preservative used. Water-borne preservative will increase the moisture content of the timber quite considerably and may raise grain. If the timber is to be glued after treatment (and re-drying) it is likely that the surfaces will have to be processed, even if lightly, which will remove some of the preservation. If a fire retardant containing ammonia or inorganic salts is to be used, then gluing by resorcinol types (and perhaps others) should not take place after treatment, nor should treatment take place until at least seven days after gluing. Certain organic solvent preservatives contain water-repellant additives which may affect the bonding characteristics.
For certain components or joints, particularly in a mass production situation, accelerated curing by radio frequency heating of the gluelines may be economical.
Alternatively, a heated environment perhaps at 60 °C can be created to reduce the setting and curing times.